1. Field of the Invention
The present invention relates to a power supply apparatus and, more particularly, to a switching regulator.
2. Description of the Related Art
In various electronic devices, in order to supply a proper voltage to an electronic circuit internally used, a step-up-type or step-down-type DC/DC converter such as a switching regulator is widely used. Such a switching regulator has a switching regulator control circuit for generating a switching signal for controlling on/off of a switching element.
As a switching signal, a PWM (Pulse Width Modulation) signal having a constant frequency for turning on/off the switching element in accordance with a pulse width is widely used (refer to Patent Documents 1 and 2). In a constant frequency system using such a PWM signal, a period since a switching element is turned on until it is turned on next is made constant to a cycle time given by the inverse number of a switching frequency. Consequently, there is a problem that the system cannot follow a load fluctuation and a fluctuation in the input voltage higher than the switching frequency, and an output is accordingly unstable.
To address an application requesting high-speed load response, a system in which the pulse width of a switching signal, that is, on-time Ton is constant and a timing at which the switching signal becomes the high level or the frequency is changed (hereinafter, referred to as constant on-time system) is considered. The constant on-time system can respond to the load fluctuation and input voltage fluctuation at higher speed as compared with the constant frequency system.
[Patent Document 1] Japanese Patent Application Laid-open No. 2003-219638
[Patent Document 2] Japanese Patent Application Laid-open No. 2003-319643
There is a case that an electronic device on which such a switching regulator is mounted needs a plurality of different voltages, and a step-down switching regulator is provided with a plurality of channels, drops the same input voltage at different dropping ratios, and outputs the dropped voltages. In this case, switching signals in a plurality of channels are turned on simultaneously. When switching elements of the channels are turned on simultaneously, an instantaneous value of input current supplied from an input power source for supplying input voltage suddenly increases. When fluctuations of the input current of the step-down switching regulator increase, the current capacitance of the input power source has to be increased or the capacitance of an input capacitor for smoothing has to be increased. Further, when the input current increases instantaneously, switching noise increases. It causes problems of erroneous operation of peripheral circuits and increase in EMI (Electro Magnetic Interference).
The problems will be considered with respect to switching regulators of two systems. The duty ratio “Duty” of the step-down switching regulator is given by the relation Vout/Vin(Duty=Vout/Vin) in a steady state. In the constant frequency system using the PWM signal, switching signals of a plurality of channels can be generated from a single oscillator, and the frequency of the plurality of switching signals can be the same irrespective of the duty ratio. Consequently, synchronization among the plurality of channels can be easily obtained, and the timing at which the switching signal is turned on can be easily shifted. On the other hand, in the case of a switching regulator of the constant on-time system, when the on-times Ton of switching signals in channels are set to be equal, the frequencies of the switching signals change as the duty ratio changes. It becomes difficult to obtain synchronization of the channels, and there is the possibility that a plurality of switching signals are turned on simultaneously. Therefore, in the case of using switching regulator of the constant on-time system in a plurality of channels, a problem of increase in the input current or EMI increases occurs.